The invention relates to a process and apparatus for thermally cracking hydrocarbons. The apparatus includes a steam superheater, a device for mixing the hydrocarbon feed with superheated steam, and a radiation block structure, in which the steam is superheated and in which the cracking reaction takes place.
In the art of thermally cracking hydrocarbons to produce olefins and diolefins, such as ethylene, propylene, butadiene, and the like, experience has shown that certain operating conditions will improve the product yield. These conditions include operating with relatively short residence times and relatively high reaction temperatures, while decreasing the partial pressures of the hydrocarbons in the reaction zone (reactor tubes). Only limited success has been achieved in the systems now being used to crack hydrocarbons.
In conventional cracking systems, the cracking reaction takes place in a cluster of individually suspended tubes, positioned within a large firebox. Such a furnace may require over 100 burners, which are usually mounted on the walls of the fire box, to transfer sufficient heat through the reactor tubes to the hydrocarbon. There are several disadvantages in such a system. One disadvantage is that all of the reactor tubes are exposed to the same flue gas temperature. This means that the maximum heat flux which can be achieved is limited by the maximum temperature at which metal breakdown of the reactor tubes generally occurs. In addition to damaging the reactor tubes, overheating can cause undesirable reactions, such as the formation of an excessively high methane content in the final product. Also, overheating causes an increase in the build-up of coke deposits on the inside of the reactor tubes.
For the reasons described above, the average heat flux over the length of the reactor tubes must be relatively low. To keep the average heat flux at a low level, the reactor tubes in a conventional cracking furnace are, of necessity, from about 50 to 100 meters in length. The long reactor tubes are not desirable because the residence time of the hydrocarbon in the reaction zone is much longer than is required for optimum cracking conditions, and the pressure drop through each tube is undesirably high.
Another process for cracking hydrocarbons, referred to as a partial oxidation-thermal cracking process, is described in U.S. Pat. No. 4,134,824. In this process, crude oil is distilled to separate the asphaltic components. The distillate is then cracked, using partial combustion gases from a methane-oil burner to generate ethylene and other products, with recycling of the asphaltic components to the burner, as fuel for the burner. Major drawbacks of this process include the necessity for separating pitch, carbon dioxide, carbon monoxide, and hydrogen sulfide from the final product.
Another procedure for cracking hydrocarbons is described in U.S. Pat. No. 4,264,435. In this process, a hydrocarbon fuel and oxygen are partially burned, at high temperatures, to generate combustion gases which contain carbon monoxide. Superheated steam is then injected into the combustion gases in a shift reaction zone, to produce hydrogen and to convert some of the carbon monoxide to carbon dioxide. The hydrocarbon feed is then injected into this mixture, in a cracking zone at a temperature of from 600.degree. to 1500.degree. C., to produce a reaction product which contains a relatively high proportion of ethylene.
This process also has several disadvantages, for example, it requires mixing tars and heavy fuel oils with oxygen to generate the burner flame for the cracking reaction. Because the cracking reaction takes place in the flame, the heavier hydrocarbons are mixed with the hydrocarbon in the cracking zone and the final product thus contains undesirable products such as methane. In addition, this process is a fully "adiabatic" operation, in which heat for the cracking reaction is supplied only by the partially burned carrier gases and steam. To supply enough heat for the reaction, the gases must be heated to very high temperatures (over 1600.degree. C.) and the ratio of carrier gases to the hydrocarbon must, of necessity, be high.